I have an external monitor and want my macbook screen to go blank or black so I can focus on the monitor instead. On my windows machine Function F5 does this.
Turn on your MacBook without the external display connected.
Close the lid and it will go to sleep.
Connect your external monitor and an external keyboard via USB.
Now wake it up by typing on the external keyboard. If you don't have a USB keyboard (you might be using a BT keyboard and mouse) you can use the Apple Remote to wake up a MacBook when its lid is closed.
The MacBook should wake up with the external monitor as its one and only monitor.
There doesn't seem to be an out-of-box way to do it in Mac OS, but with this third-party plugin https://github.com/Eun/DisableMonitor does the trick! Just download and run the app form the button in the readme shown, and an icon will show in the menu bar like just like Airplay does, and from it you pick the monitor or monitors shown and whether the Disable it.